Where is Dennis Crowley now?

When most people want to tell friends where they are, they send a text message. When Dennis Crowley wanted to do so, he created Foursquare.

The location-based social network lets users "check in" at different spots to alert friends of their whereabouts. Frequent users can also get deals, receive virtual badges based on their check-ins and even become mayors, albeit digital ones. Foursquare officially launched at the South by Southwest festival in March 2009, where it was swiftly adopted by 10,000 attendees looking to keep tabs on each other.

"Even then, I kept saying we'd never hit a million users," said Mr. Crowley.

Today, Foursquare has 7 million users, more than $20 million in funding and over 50 employees, up from five in January 2010. Mr. Crowley, who just oversaw the launch of version 3.0, hopes the startup will turn a profit next year. Foursquare is already fielding requests from companies like Pepsi and Bravo to get in on the action.

Mr. Crowley has always had a thing for social networking. In 2000, the Massachusetts native founded Dodgeball, a Foursquare predecessor. (Google acquired the site but later shut it down, a move that helped drive Mr. Crowley to launch Foursquare.) He also built a sharable database of fellow students—a prescient undertaking in 2002—while getting his master's degree at New York University.

"I remember seeing that and hearing that a first-semester student had created it," said Clay Shirky, associate arts professor at NYU's Interactive Telecommunications Program. "I went and introduced myself to him."

The Foursquare co-founder, who still wears jeans and sneakers at the office, has but one complaint about life at the top.

"I used to snowboard 30 days a year," he said. "Now it's down to eight."